A battery separator is used to separate the battery's positive and negative electrodes or plates in order to prevent an electrical short. Such a battery separator is typically microporous so that ions may pass therethrough between the positive and negative electrodes or plates. In lead/acid storage batteries, either automotive batteries or industrial and/or deep cycle batteries, the battery separator is typically a microporous polyethylene separator having a backweb and a plurality of ribs standing on the backweb. See: Besenhard, J. O., Editor, Handbook of Battery Materials, Wiley-VCH Verlag GmbH, Weinheim, Germany (1999), Chapter 9, pp. 245-292. The separators for automotive batteries are typically made in continuous lengths and rolled, subsequently folded, and sealed along the edges to form pouches or envelopes that receive the electrodes for the batteries. The separators for industrial (or traction or deep cycle storage) batteries are typically cut to a size about the same as an electrode plate (pieces or leaves).
In a typical lead/acid microporous polyethylene battery separator, the separator has a ribbed face (i.e., with the primary ribs) and a back face (i.e., without ribs or a plurality of small or secondary ribs). The negative electrode (or plate) may be placed adjacent to the back face, while the positive electrode (or plate) may rest on the ribs of the ribbed face. U.S. Pat. No. 3,917,772 illustrates a method of making a lead/acid battery separator sheet from a plastic material and is hereby incorporated by reference herein. In this method, the sheet is calender molded to form ribs and/or projections. Referring to FIGS. 2 and 3 of U.S. Pat. No. 3,917,772, ribs 29 and 31 have a solid profile. Referring to FIGS. 4 and 5 of U.S. Pat. No. 3,917,772, discrete projections 45 and 49 are formed by rounded pits in the calendering rolls 12 and 13. U.S. Pat. No. 4,000,352, which is hereby incorporated by reference herein, illustrates a lead/acid battery separator characterized by an interrupted pattern of discrete separator projections standing on a backweb. Each projection has a circular or oval shape. U.S. Pat. No. 5,558,952, which is hereby incorporated by reference herein, illustrates a lead/acid battery separator having a plurality of discrete ribs with no intermediate connecting walls. U.S. Pat. No. 5,716,734, which is also hereby incorporated by reference herein, illustrates a lead/acid battery separator having a plurality of ribs, each with a solid profile. PCT Publication WO 01/13442, which is hereby incorporated by reference herein, illustrates a lead/acid battery separator having at least one vertical rib and a plurality of studs. The studs are truncated cones, and their bases are flush with the backweb. The ribs have a solid profile.
The typical separator for lead/acid batteries is made from polyethylene (“PE”) resin. The manufacturing of the PE separator used in lead/acid batteries, like flooded lead acid batteries, may include extruding a mixture of precipitated silica, PE, such as Ultra High Molecular Weight polyethylene (UHMWPE) or some other type of PE, and mineral oil, along with some other minor ingredients into a profile sheet or film. This profile sheet is then solvent extracted where the bulk of the mineral oil is removed or extracted to create porosity in the sheet or film. The last step of the process is often called finishing, where the profile sheet is mechanically adjusted to the desired width, overall thickness and length.
The aforementioned process produces a PE separator that may be used in a variety of applications including automotive, stationary and deep cycling (or traction or industrial) batteries. In general, the automotive separators are thinner in backweb and overall thickness as compared to those separators used in deep cycling and/or industrial batteries. The following table, Table 1, highlights the typical differences in mechanical dimensions of the automotive battery separators versus the deep cycling and/or industrial battery separators.
TABLE 1AutomotiveDeep CycleUnitsSeparatorSeparatorBackweb(mils) 6 to 1012 to 25ThicknessOverall(mils)12 to 65 65 to 140ThicknessRib Base Width(mils)12 to 5035 to 80Extraction Time(seconds)45 to 75180 to 300To better understand the numbers in Table 1 above, FIG. 1, which shows a typical deep cycle battery separator, also shows backweb 20 having a certain backweb thickness (or height) and rib 22 having a certain rib base width (where rib 22 meets backweb 20). The separator shown in FIG. 1 has an overall thickness that includes the backweb thickness as well as the height of the rib 22 above the backweb.
Because of the structural differences between a typical deep cycle battery separator and a typical automotive battery separator, the deep cycle battery separator usually takes longer to make, for example, it may take approximately 3 to 5 times longer in the extraction process to remove the oil to the proper level than a typical automotive separator. The time in the extraction process is defined by diffusion principles where the oil is removed from the separator. As the deep cycling separator has a thicker backweb thickness and larger ribs, the rate of diffusion of the oil out of the separator is substantially longer than the time required to extract oil out of the automotive separator.
As such, there is a need or desire to produce a deep cycling separator or a separator for a deep cycle storage battery or an industrial or traction battery with the desired backweb thickness and overall thickness as well as rib shape that is useful for a deep cycle battery separator, while also reducing the manufacturing time for such a separator, reducing the extraction time for such a separator and/or simplifying the manufacturing process for such a separator.